I have the same situation this dosn't work with cisco ioses. you need the ios with ending YB or XL
The ios with ending .121-5YB works with pppoe the isdn dosn't work. the ios wirh ending .121-4 XL crashed the AAA Server when the router is starting up, you can not login on the router via console or telnet. in my situation i using a standard ios .122-6a and connect an Zyxel R642R-I on my second Fasteth0/0. This Router is directly connectet to my ISP over adsl. This works.